In order to improve the performance and efficient usage of our MARS system, the server produces a warning whenever an inefficient access pattern is detected. A typical inefficient request pattern usually involves repeated access to elements of data which are stored in a single tape using separate MARS requests. This pattern forces multiple mounts of the tape (which is a very expensive operation for a tape-based mass storage system) that could be avoided if a single combined larger request were to be used.
We would recommend you to read Guidelines to write efficient MARS requests, but please do contact our ServiceDesk via email (servicedesk@ecmwf.int) or our support portal if you need any further advice.